Since its first marketing approval in 1984, metsulfuron-methyl has become one of the most commercially important herbicides for control of a wide range of grass and broad-leaved weeds in crops including wheat, barley, oats and triticale through post-emergence application (see The Pesticide Manual, Sixteenth Edition, C. MacBean ed., British Crop Protection Council, Hampshire, UK, 2012, pp. 782-3). Like other sulfonylurea herbicides, the mode of action of metsulfuron-methyl involves inhibition of the enzyme acetolactate synthase (ALS) found in plants but not animals, and therefore metsulfuron-methyl provides a valued combination of excellent efficacy against weeds with low use rates and very low toxicity to animals.
Since its introduction, metsulfuron-methyl has typically been formulated in herbicide products as solid compositions, such as granules, particularly water-dispersible or water-soluble granules, and tablets. Not only can granules and tablets be readily dispensed, but metsulfuron-methyl generally has excellent storage stability in solid compositions.
Although metsulfuron-methyl controls a wide range of weeds, combination with other active ingredients, particularly other herbicides, can be desirable to provide a broader spectrum of protection or for resistance management. Solid compositions are suitable for metsulfuron-methyl in combination with other high-melting active ingredients, but including active ingredients that are liquids or low-melting in a solid composition can be difficult.
Concentrated liquid compositions can readily include not only active ingredients that are liquids or low-melting but also high-melting active ingredients such as sulfonylurea herbicides, which generally are mostly present in the form of solid particles dispersed in a liquid carrier. Such liquid concentrate formulations can be easily measured and poured, and when diluted with water typically give easily sprayed aqueous solutions, emulsions or dispersions.
However, compared to solid formulations, liquid formulations of sulfonylureas are more prone to certain problems. When dissolved or dispersed in a liquid carrier, even a carrier comprising a nonaqueous solvent, sulfonylurea herbicides may be susceptible to decomposition. Also, crystal growth can occur during storage of concentrated liquid formulations in which active ingredients are dissolved or dispersed, and suspended active ingredients are prone to settling out, so that obtaining stable liquid formulations is challenging.
Because of its high herbicidal activity, metsulfuron-methyl is typically applied at very low rates, e.g., 4-8 g/ha. Accordingly when combined with much higher use rate herbicides in a concentrated formulated composition, metsulfuron-methyl must be in low concentration to accommodate a sufficiently high concentration of higher use rate herbicides in the composition. However, it has now been found that although metsulfuron-methyl is sufficiently stable at high concentration in liquid carriers comprising one or more fatty acid esters of C1-C4 alkanols in the presence of one or more surfactants having an emulsifier property, at concentrations of 1% or less, metsulfuron-methyl is susceptible to undergoing decomposition to a commercially undesirable extent during storage of the composition.
Remarkably, as described herein, a surprising means of stabilization of low concentrations of metsulfuron-methyl in such a liquid composition has now been discovered, thus providing a new stabilized liquid composition for dilution with water and application to undesirable vegetation to be controlled.